Polar codes are a new type of channel coding proposed by Professor Arikan of Turkey in 2008. The polar code is designed based on channel polarization, and is the first constructive coding scheme that can be proved through a strict mathematical method to achieve a channel capacity. The polar code is a linear block code. Its generator matrix is FN, and its encoding process is x1N=u1NFN, where u1N=(u1, u2, . . . , uN) is a binary row vector with a length of N (N is called a mother code length), FN is an N×N matrix, and FN=F2⊗(log2(N)), where
            F      2        =          [                                    1                                0                                                1                                1                              ]        ,and F2⊗(log2(N)) is defined as a Kronecker product of log2 N matrices F2.
In the encoding process of the polar code, some bits in u1N are used to carry information, and are referred to as information bits, and an index set of these bits is denoted as I; and the other bits are set to fixed values that are pre-agreed on by a receive end and a transmit end, and are referred to as frozen bits, and an index set of the frozen bits is represented by Ic, a complement of I. The information bit index set I is selected by using the following method: obtaining, by using a polar code construction algorithm, a channel error probability Pe(i) or a channel capacity estimation C(i) corresponding to a bit with a sequence number i, and selecting K sequence numbers with smallest Pe(i) values or largest C(i) values, to construct the set I.
The channel error probability Pe(i) or the channel capacity estimation C(i) is related to channel reliability. Usually, the channel reliability may be calculated by using formula (1):
                                          W            1                          2              i                                =                      [                                          W                1                                  2                                      i                    -                    1                                                              ,                              W                                                      2                                          i                      -                      1                                                        +                  1                                                  2                  i                                                      ]                          ,                              W            1            2                    =                      [            01            ]                                              (        1        )            
where W2i−1+12i=W12i−1+βi, βi=(2i−1)1/4, W12i is channel reliability of a channel sequence number 1-to-2i, and i is a channel sequence number.
In addition, the channel reliability may alternatively be calculated by using formula (2):
                              W          i                =                              ∑                          j              =              0                                      n              -              1                                ⁢                                    B              j                        ⋆                          2                              j                ⋆                                  1                  4                                                                                        (        2        )            
where iBn−1Bn−2 . . . B0, Bj∈{0,1}, j∈{0,1, . . . n−1}, and Bn−1Bn−2 . . . B0 is a binary representation of i.
After the channel reliability is calculated in the foregoing manner, an obtained channel sequence is fixed, and therefore a determined position set of information bits is relatively monotonous.